In recent years, along with a tendency to a high brightness and whitening of a light-emitting diode (hereinafter referred to as LED) element, a light-emitting device employing a LED element has been used for backlights of mobile phones or liquid crystal TVs or liquid crystal displays, generic illumination, etc. Accordingly, peripheral components of a LED element are also required to have higher performance. For example, as a substrate to mount a LED element, one made of a resin material is used. However, such a resin substrate is likely to be deteriorated by a heat or light accompanying higher brightness of a LED element. Accordingly, a study has been made to use a substrate made of e.g. an inorganic insulating material.
Such an inorganic insulating material may, for example, be a ceramics such as alumina or aluminum nitride, or a low temperature co-fired ceramics (LTCC) which is a composite of glass with a ceramics powder such as alumina. LTCC is one fired usually at a temperature of from about 800 to 1,000° C., which is lower than the firing temperature for usual ceramics and is prepared by laminating a prescribed number of green sheets made of glass and a ceramics powder (such as an alumina powder or a zirconia powder), integrating them by hot pressing, followed by firing. An inorganic insulating substrate made of such inorganic insulating materials has a higher durability against a heat or light as compared with a resin substrate and thus is prospective as a substrate for mounting a LED element.
On the surface of an inorganic insulating substrate, a thick conductor layer is formed which is prepared by printing a paste composed mainly of a conductor metal such as silver (Ag) or copper (Cu), followed by firing. And, among such thick conductor layers, particularly terminal portions (electrodes) to be connected to the element are subjected to lamination plating (Ni/Au plating) of nickel (Ni) plating and gold (Au) plating to maintain the wire bonding property, the adhesion strength and the weather resistance. By such Ni/Au plating, sulfurization resistance is imparted to prevent a color change by a reaction of the thick conductor layer with a sulfur (S) content in the air, etc.
However, in recent years, a substrate to mount a LED element or the like, is required to have sulfurization resistance, and with conventional plated thicknesses (Ni-plated thickness of from 3 to 5 μm/Au-plated thickness of from 0.1 to 0.3 μm) required for the wiring bonding portions, there has been a problem that a color change to black is observed at the Ni/Au-plated portion in a sulfurization test in accordance with JIS-C-60068-2-43, thus failing to pass the sulfurization test.
By a research conducted by the present inventor, it has been found that such a color change of the Ni/Au-plated portion is attributable to formation of nickel sulfide by a Ni-plated layer exposed to the surface. That is, a thick conductor of Ag or the like has grain boundary void spaces or surface irregularities, and even if a Ni-plated layer is formed, irregularities will remain on its surface, and even if Au-plating is applied as the uppermost layer, if it cannot completely cover the Ni-plated layer, the Ni-plated layer as an underlayer of the Au-plated layer will be exposed on the surface. And, such an exposed Ni-plated layer and a sulfur (S) content will react to form black-colored nickel sulfide.
Heretofore, as a technique to prevent such sulfurization (color change) of connection terminal portions, a method of applying a protective coat by e.g. a silicone resin on the Ni-plated layer, a method of forming a thick Au layer by paste printing instead of plating, or a method of increasing the thickness of the Au-plated layer, has been known. Further, particularly in the case of a LTCC substrate, a method has been known wherein the particle size of the Ag powder to be used as a conductor to constitute the thick conductor layer is made small to improve the sintering property thereby to reduce grain boundaries.
However, the method of forming a thick Au layer or increasing the thickness of the Au-plated layer has had a problem that the production cost increases substantially. Further, in the case of the LTCC substrate, there has been a problem such that if the sintering property is improved by reducing the particle size of the Ag powder, timing in shrinkage by firing will not meet with the substrate whereby the substrate is likely to undergo warpage.
Further, as a technique to treat the surface of the conductor layer formed on a LTCC substrate to improve the plating property, a method has been proposed wherein prior to a plating step, the surface of a LTCC substrate is subjected to wet blast treatment, and glass exposed on the surface of the conductor layer is removed (e.g. Patent Document 1).
However, by this method, it was not possible to increase the sulfurization resistance by removing the grain boundary void spaces or surface irregularities of the thick conductor (Ag) layer. That is, although Patent Document 1 does not disclose the conditions for the wet blast treatment in detail, the blast treatment to remove glass is one to break and remove glass as a hard substance in a short time by blasting. Under blast treatment conditions for such a purpose, it has been difficult to fill spaces among the conductor (Ag) particles. And, it has been difficult to remove the surface irregularities of the thick conductor (Ag) layer thereby to planarize (smooth) the layer surface to such an extent to make it possible to completely cover it with the Au-plated layer having a usual thickness.